Since our first recording of the magnetic field of an isolated nerve bundle in 1979, we have conducted a unique series of measurements of magnetic fields produced by cellular action currents in nerves and muscles. During the preceding 27 months of NIH support, we have pursued three aspects of biomagnetic measurements: improving the instrumentation, developing mathematical models relating the magnetic field to cellular action currents, and performing experiments to test our models and to explore the capabilities of the technique. The magnetic technique has now advanced to the point that it can be used to determine both the intracellular action current and the transmembrane action potential without the need to penetrate the axon, and it provides a precise means for determining axoplasmic resistivity insingle axons. There are a large number of problems in both basic and clinical research that we can address. Existing intraoperative electrical techniques to assess the extent of injury to a traumatized peripheral nerve are limited by the need to lift the nerve with hook electrodes and dry if off. Magnetic techniques allow measurement of the nerve lying in its own tissue bed while immersed in saline, thereby reducing the risk of damage to the nerve and increasing the measurement reproducibility and accuracy. Chronically implanted toroids can be used to monitor the regeneration of a nerve and possibly to control a prosthetic device, without the problems of electrochemical effects and connective tissue growth at the electrode-tissue interface. Compared to electrical techniques, stimulus artifacts may prove to be less of a problem with magnetic stimulation and recording. Basic research on axons, nerve bundles, skeletal muscle, and smooth muscle is hampered by the inability to make direct electrical measurements of intra- and intercellular currents without detailed assumptions regarding resistivities. Magnetic techniques overcome this. We propose to determine whether magnetic measurements of cellular action currents can eliminate some of the practical and fundamental problems of electrical measurements. Animal experiments will be used to determine the capabilities of magnetic assessment of electrical properties and conduction in bundled nerves and skeletal muscle, to study current propagation in septated nerves and smooth muscle, and to explore magnetic nerve stimulation. We will begin intraoperative neuromagnetic measurements on humans. Successful completion of this research may result in the development of new clinical tools and the extension of our understanding of cellular action currents in multi-cell systems.